U.N. rapporteur Philip Alston calls on the U.S. to put the military in charge of the targeted killings program, which is shrouded in secrecy under the CIA and has prompted accountability questions.

Reporting from Geneva — The campaign of CIA drone strikes against suspected militants in Pakistan has made the United States "the most prolific user of targeted killings" in the world, said a United Nations official, who urged that responsibility for the program be taken from the spy agency.

Philip Alston, a New York University law professor who serves as the U.N.'s special rapporteur on extrajudicial killings, made the comments Wednesday as he released a report on targeted killings. The report criticizes the U.S. for asserting "an ever-expanding entitlement for itself to target individuals across the globe" in its fight against Al Qaeda and other militant groups.

Alston acknowledged that the right to self-defense may justify drone strikes in Pakistan, where the planners of the Sept. 11 attacks are thought to have fled. But he questioned whether that right extended to other countries where links to the attacks are more remote, such as Yemen or Somalia. He urged the U.S. to be more open about the program.

He also expressed concern about the precedent set by the U.S. program. Many other countries are seeking drone technology and when they obtain it, they are likely to copy U.S. tactics, he said.

Alston is scheduled to present his findings Thursday to the U.N. Human Rights Commission in Geneva. They represent one of the most critical assessments to date of U.S. drone strikes, a tactic that has been stepped up significantly under the Obama administration. U.S. officials have credited the program with inflicting severe blows to militant groups.

As the drone attacks have expanded, they have attracted increasing criticism from human rights organizations and international legal scholars, some of whom claim aspects of the program violate international law. Critics also contend that the attacks risk a backlash in Pakistan and other countries where they are carried out.

The U.S. uses the unmanned aircraft extensively over Afghanistan and Iraq, as well, but those are primarily reconnaissance flights and are run by the military. The U.S. does not officially acknowledge the CIA program that focuses on Pakistan. Still, Obama administration officials dismissed much of Alston's criticism of the CIA program.

"We have a way to get at dangerous terrorists operating in areas otherwise inaccessible to the central government or to conventional military units. It's effective, exact and essential," said a U.S. counter-terrorism official who was not authorized to comment publicly and spoke on condition of anonymity.

In a statement, CIA spokesman Paul Gimigliano said: "Without discussing or confirming any specific action, this agency's operations are, of course, designed to be lawful and are subject to close oversight within our government. The accountability is real, and so is the fidelity to American policy."

The White House deputy press secretary, Bill Burton, would not comment on the report's findings but said that the president "is focused on making sure that he's doing everything in his power to protect the security of our country."

Alston's report discusses targeted killings by several countries, including Russia, Israel and Sri Lanka. But he focused on the U.S. use of drones. Many other countries are seeking to acquire unmanned aircraft, the report says, because the aircraft "permit targeted killing at little or no risk."

"This strongly asserted but ill-defined license to kill without accountability is not an entitlement which the United States or other states can have without doing grave damage to the rules designed to protect the right to life and prevent extrajudicial executions," Alston said.

Current and former U.S. officials familiar with the program took issue with several of Alston's findings. Two officials said the Obama administration has limited the target list outside Afghanistan and Pakistan to members of Al Qaeda and allied groups, a tighter standard than existed during the Bush administration.

Only a handful of attempts of any kind have been made to kill suspected militants outside Afghanistan, Pakistan and Iraq since 2001, the officials said. One occurred last year in Somalia, where U.S. special operations troops in a helicopter killed Saleh Ali Saleh Nabhan, a senior Al Qaeda operative, U.S. officials familiar with the operation have said.

Under President Obama, the number of airstrikes in Pakistan has increased to an average of more than two a week, in part because the CIA was given authority in 2008 to carry out strikes against individuals deemed to be a threat to the United States, even when the U.S. does not know their names or has only fragmentary information about their intentions.

U.S. officials say that precautions are taken to avoid civilian casualties and that each target is carefully vetted, sometimes by hours of aerial surveillance that is matched against other intelligence.